Devices for urging doors or the like into either closed or open positions are well-known in the art. It is frequently desired that a door should be kept in a closed position for safety, security or other reasons, and this is particularly true in elevator systems with respect to the landing doors.
In elevator systems, sliding doors are placed at each landing to secure the elevator hoistway during periods when the elevator car is not present at the landing. In this particular application, the mechanism for causing both the landing and car doors to open is mounted on the elevator car and moves vertically therewith. Upon arrival at a landing to be served, the elevator car doors and landing doors are engaged, and the opening mechanism drives both into the open position. Although this same mechanism also causes the doors to return to their closed positions upon completion of the loading and unloading of passengers, it is desirable that the landing doors include independent means or urging the doors into a closed position.
For space and other considerations, it is common to use a cable mechanism for driving the landing doors. One such mechanism known in the art utilizes a winding drum upon which the drive cable is wound and unwound as the door is slid closed and opened. A torsional spring, connected to the winding drum, provides an increasing torque on the drum as the cable, which is connected at one end to the landing door, is pulled off during opening. When the landing door is released, the spring winds the cable back on the drum, thereby pulling the landing door into the closed position.
One problem, however, with this prior art arrangement is that the torsional spring does not provide a uniform torque to the drum over the range of travel of the door. As will be apparent to those skilled in the art, a torsional spring will deliver the least torque when the door is in the closed or first position, and the most torque when the door is in the open or second position and the cable has been fully unwound from the winding drum. This variation in torque produces a similar variation in the restoring force imparted by the cable to the door. In order to open hoistway doors equipped with the torsional spring and cable apparatus of the prior art, it is necessary to provide a door opening mechanism with sufficient strength to overcome the highest force generated by the door closing apparatus. This results in a door opening mechanism which is much more powerful, heavier, and costly than desirable.
What is needed is a door closing device which provides a uniform closing force over the length of travel of the moving door, but which also retains the space and flexibility advantages of the prior art winding drum and cable device.